Overload relays are inexpensive, but the results of selecting the wrong one for the application can be catastrophic for your motor, process, or both.
When it comes to manufacturing, motors make the world go 'round. This makes proper motor protection mission-critical. Enter, the overload relay. Overload relays protect a motor by sensing the current going to the motor. Many of these use small heaters, often bi-metallic elements that bend when warmed by current to the motor.
When current is too high for too long, heaters open the relay contacts carrying current to the coil of the contactor. When the contacts open, the contactor coil de-energizes, which results in an interruption of the main power to the motor. These contacts do not affect control power (which is often 120V), so don't assume an absence of potentially lethal current without a proper lockout/tagout.
Types of relays. Overload relays and their heaters belong to one of three classes, depending on the time it takes for them to respond to an overload in the motor. The overload relay itself will have markings to indicate which class it belongs to. These include Class 10, 20, and 30. The class number indicates the response time (in seconds). An unmarked overload relay is always Class 20. Typical NEMA-rated overload relays are Class 20, but you can adjust many of them about 15% above or below their normal trip current. IEC relays are usually Class 10, and you can usually adjust them to 50% above their normal trip current.
When replacing overload heaters, always replace the entire set. Why? Because there is some damage to the remaining two heaters, and you may wind up playing a game of musical chairs as they take turns failing prematurely.
Heater selection. Selection is straightforward, if you can use the identical brand and size. However, this is not always possible. If you must select a different heater, refer to the manufacturer's selection tables. Your choice will depend on the full load amperage (FLA) of the motor, and the motor starter you use.
For example, suppose you have to select replacement overloads for a 100-hp motor drawing 162A at full load. Let's say you have a NEMA Size 5 controller. We'll use an excerpt from an actual manufacturer's index of tables (see table, above). This example shows you how the selection criteria interact. All manufacturers' indexes and tables are easy to use, but let's do a dry run with this example.
To make the proper selection from this manufacturer, start with the bulletin number (left column). This leads you to the proper table (right column). In this case, the index tells you to use Table Number 147 for the 506 Series A. On the manufacturer'sTable 147, you should locate the FLA of the motor in the column for NEMA Size 5 controllers. If the FLA of your motor does not exactly match the FLA of the table, just choose the closest heater element: W38 in this case. This assumes your motor and controller operate at the same temperature. If there is a small temperature difference (less than 15 degrees F) between the motor and the controller, choose a heater based on the controller. Choose the higher heater number if the controller is warmer than the motor. Choose the lower heater number if the controller is cooler than the motor. If there is a significant temperature difference (15 degrees F or more) between the motor and the controller, consult the manufacturer or supplier. Reliable overload protection for the motor will require further adjustments to the selection process.
Editor's note: Don't confuse motor overload protection with circuit breaker protection, because they serve two different purposes. Your motor overload protection will interrupt power to the motor to protect only the motor. Your circuit breaker will open to protect the power distribution to the motor. You must do both, and no single device accomplishes both. You should size your circuit protection to protect the feeders, and coordinate the motor feeder circuit protection with the upstream breaker scheme. —M.L.L.